feat: harden console sessions + complete protocol support

- single-use session tokens (atomic ConsumeSessionToken) + user-bound tunnel (actor must match session opener)
- wire VNC end-to-end (stash/consume password, connect+tunnel, frontend password field)
- passphrase-protected SSH keys: passphrase_enc on Key model, capture on upload, decrypt + pass to guacd

server/internal/grpc/server.go

@@ -57,7 +57,8 @@ func (s *vantageServer) UploadGeneratedKey(ctx context.Context, req *pb.UploadKe
 		return nil, status.Errorf(codes.Unauthenticated, "invalid agent token")
 	}
 
-	key, err := services.CreateKey(req.Label, req.PublicKey, "generated", srv.ServerID, req.PrivateKey)
+	// Agent-generated keys carry no passphrase over the wire (proto has no field).
+	key, err := services.CreateKey(req.Label, req.PublicKey, "generated", srv.ServerID, req.PrivateKey, "")
 	if err != nil {
 		return nil, status.Errorf(codes.Internal, "failed to store key: %v", err)
 	}